This invention relates generally to wafer polishing and, particularly, to a method of conditioning a pad for use in polishing semiconductor wafers with a double side or single side polishing machine.
Most processes for fabricating semiconductor electronic components start with monocrystalline, or single crystal, semiconductor material in the form of wafers. Semiconductor wafers are produced by thinly slicing a single crystal ingot into individual wafers with a cutting apparatus, such as a wire saw or inner diameter saw. The as-cut wafers undergo a number of processing operations to shape them, reduce their thicknesses and remove damage caused by the slicing operation. In addition, the wafers undergo chemical-mechanical polishing to planarize their surfaces. This polishing technique involves rubbing each wafer with a polishing pad in a solution that contains an abrasive and chemicals to produce an extremely flat, highly reflective and damage-free wafer surface. One such polishing solution, or slurry, includes a colloidal silica and an alkaline etchant. The polishing pad is, for example, a polyurethane impregnated polyester felt having a thickness between about 1.5 mm and 2.0 mm.
In determining the quality of a processed semiconductor wafer, the flatness of the wafer is a critical parameter to customers since it has a direct impact on the subsequent use and quality of semiconductor chips diced from the wafer. A number of parameters determine the wafer flatness, including a GBIR (Global Backside Indicated Reading) measurement. The GBIR measurement represents the difference between the highest point on a top surface of the wafer with respect to a reference plane parallel to the back side of the wafer. In this instance, the wafer is mounted on a vacuum chuck that translates any surface variations on the back side of the wafer to the front side of the wafer for measurement. ADE Corporation of Westwood, Mass. sells non-contacting electric-capacity type sensors for characterizing wafer geometry and measuring flatness under the trademarks UltraGage.RTM. 9500 and Galaxy AFS-300.TM..
To maximize throughput in the preparation of semiconductor wafers, a polishing machine polishes many wafers simultaneously. Such a machine typically holds 5 to 30 wafers, depending on their size, in carriers. The machine moves the carriers relative to a rotating circular turntable, or platen, for polishing. The platen is typically cast iron and overlaid with a polishing pad. The machine dispenses a stream of polishing slurry to a surface of the pad while the pad is pressed against the wafers. Single-side polishing machines have one platen for polishing a surface of the wafers, while double-side polishing machines have two platens for polishing the top and bottom surfaces of the wafers simultaneously. Both the platen and polishing pad must be extremely flat to ensure that polished wafers are likewise extremely flat. During polishing, the wafer carriers and platen usually rotate in opposite directions for a predetermined time, a typical duration being about 30 to 80 minutes.
Unfortunately, conventional polishing machines usually produce highly concave (dished shape) wafers the polishing pads are new. These wafers typically have an unacceptable global flatness, GBIR, of approximately 1.5 .mu.m or more. One procedure for preventing unduly concave wafers after new polishing pads have been installed on a polishing machine is to condition the pads by performing 10 to 20 dummy runs before actual polishing runs begin. In a dummy run, which takes about one hour per run, the new pads are used to polish dummy wafers (e.g., wafers rejected for various reasons). Under the conventional conditioning procedure, approximately 10 to 20 hours of dummy runs are needed to condition the newly installed polishing pads before relatively flat wafers can be produced by the polishing machine. For this reason, a method is desired for economically and quickly conditioning new polishing pads without numerous, expensive and time-consuming dummy runs.